Revert of df generation: single allocation with calloc (patchset #2 id:20001 of https...

Revert of df generation: single allocation with calloc (patchset #2 id:20001 of https://codereview.chromium.org/1544983004/ )

Reason for revert:
Crashing multiple bots, e.g.
https://uberchromegw.corp.google.com/i/client.skia.android/builders/Perf-Android-GCC-Nexus7-GPU-Tegra3-Arm7-Release/builds/3525

Original issue's description:
> df generation: single allocation with calloc
>
> The dfStorage DFData allocation can never fit in its stack space: 5px padding on each side always implies at least a 10x10 DFData allocation, but the stack space only fits 64 DFData.
>
> So we've always been spilling to the heap.
>
> If we're going to spill to the heap, we might as well allocate/free all our temporary memory in one block, and since we want it zeroed, might as well calloc.
>
> So in practice this replaces 1-2 malloc, 1-2 free, and 2 bzeros with 1 calloc and 1 free.
>
>
> BUG=skia:4729
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1544983004
>
> Committed: https://skia.googlesource.com/skia/+/12204d90337656542a42fa0fcccb7bec13af0cce

TBR=jvanverth@google.com,mtklein@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:4729

Review URL: https://codereview.chromium.org/1556263002